The present invention relates to a method and apparatus for the spectrochemical analysis of a sample in which a solid-state array detector is used to detect radiation of spectrochemical interest. In particular, the method and apparatus relate to utilisation of the solid state array detector.
Throughout this specification, the terms xe2x80x9clightxe2x80x9d and xe2x80x9cradiationxe2x80x9d have been used interchangeably.
It is known that chemical analysis of samples can be accomplished by a variety of spectroscopy-based techniques. For example, the amount of various chemical elements in a sample can be ascertained by optical emission spectrometry or by atomic absorption spectrophotometry. The concentration of various chemical species in a sample can be ascertained by ultravioletxe2x80x94visible absorption spectrometry or infrared absorption spectrophotometry, or by ultraviolet-visible fluorescence spectrophotometry. These are only a few examples of spectroscopy-based chemical analysis techniques.
Apparatus for spectroscopy-based chemical analysis typically operates by measuring the intensity of light either as a function of wavelength or at one or more specific wavelengths. This may be done with a monochromator and a single detector collecting intensity data for each wavelength of interest in a serial fashion, but it is also possible to collect light intensity data for more than one wavelength simultaneously. Because of the greater time efficiency offered by simultaneous measurement, this approach is increasingly favoured for practical applications.
Modern simultaneous spectroscopic measurement apparatus typically includes an optical polychromator together with a solid state electronic detector device incorporating an array of optical sensor elements. The detector can be, for example, a charge-transfer device such as a charge-injection device (CID) or a charge-coupled device (CCD). A polychromator that is able to disperse the light in two dimensions (for example an echelle polychromator) can be employed, in which case a 2-dimensional array of optical sensor elements can be used with advantage as a detector. Alternatively a polychromator that provides dispersion in one dimension only (such as a single-grating-based polychromator) can be utilised, and a linear array detector used. The 2-dimensional approach offers better wavelength resolution for a given wavelength range and so is favoured for chemical analysis applications, particularly for elemental analysis by optical emission spectrometry. Compared to linear detectors, such two-dimensional arrays are more compact. The need for several detectors to cover the focal plane of the spectrometer can be avoided by the use of an appropriate two-dimensional array detector.
Elemental analysis typically involves operation at optical wavelengths extending from the visible to the far ultraviolet, which places limitations on the types of detectors that can be used. Primarily, the detector must be efficiently responsive to radiation across this range of wavelengths. Solid-state detectors of various types are known to be suitable for this application, for example charge transfer devices, both CIDs and CCDs, are known to be useful. Such devices are described, for example, in the book xe2x80x98Charge Transfer Devices in Spectroscopyxe2x80x99, J. V. Sweedler, K. L. Ratslaff and M. B. Denton, eds., VCH Publishers, Inc., New York, 1994. ISBN 1-56081-060-2. CCDs are discussed in xe2x80x98Scientific Charge-Coupled Devicesxe2x80x99, J. R. Janesick, SPIE Press , Bellingham, Wash., 2001, ISBN0-8194-3698-4.
A specific example of such a detector is the CCD detector disclosed by Zander et al. in U.S. Pat. No. 5,596,407. This has a number of optically sensitive sites, generally referred to as pixels, that are distributed in a precise geometric arrangement over the surface of the detector to map accurately the optical image from the polychromator. Each optically sensitive site or pixel is capable of converting the energy of incoming light to free electrons, which are stored at the optically active site. The number of electrons, and thus the total charge, accumulated within each pixel will depend on the light intensity incident on that pixel and the time for which the pixel is exposed to said light, said time being usually referred to as the integration time.
Measuring the optical intensity therefore involves determining the amount of charge built up over a known integration period. In order to do this it is necessary first to collect the charge and then to transfer the charge accumulated at each pixel to appropriate readout electronics. Two principal ways of carrying out this process are available. The first, used in the detector disclosed by Zander et al. in U.S. Pat. No. 5,596,407, duplicates each optically active pixel with an optically inactive pixel. The first step in the readout process is a parallel transfer operation that transfers the charge from each row of active pixels to the corresponding row of inactive pixels so that these inactive pixels are used as the shift register nodes. The charge is then stepped through one optically inactive pixel to the next to readout electronics at the end of the row. The second approach uses the optically active pixels themselves as shift register nodes, so that with each move operation the charge on every pixel moves to the next pixel along, with the charge of the first pixel moving to the readout circuit.
Both approaches have their attendant advantages and disadvantages. The second approach has the advantage that most of the surface area of the CCD can be covered by active pixels, thus maximising the light sensitivity of the whole device. It also avoids the need for any secondary structure. That is, this approach provides more efficient utilisation of available light in spectroscopic applications. It also permits the use of relatively inexpensive, off-the-shelf detectors, or of custom-designed detectors that can be fabricated relatively inexpensively using the same technology as that used for the off-the-shelf detectors.
The disadvantage of the second approach is that the pixels continue to accumulate electrons generated by any incoming light during the readout process. As a consequence, as the charge from one pixel moves through other pixels on its way to the readout circuitry, it accumulates additional charge, the amount of which depends on the light intensity incident at each of those other pixels and the speed of charge transfer. This has the effect of smearing the resultant image data, which is unacceptable in a spectroscopy application.
The smearing problem does not occur with the first approach, since the readout occurs through optically inactive pixels. However this advantage is at the expense of the need for a secondary structure and a reduced overall light sensitivity, due to the loss of that proportion of the detector""s surface area that is taken up by the inactive pixels. Additionally, detectors of this type have to be custom-designed and custom-built, and are consequently expensive.
In spectrochemical applications it is common for a sample to emit extremely intense radiation at certain wavelengths and to emit extremely feeble radiation at other wavelengths, depending on the amount of specific chemical elements present in that sample. To extract the required chemical information it is often necessary to measure both extremely feeble and extremely intense radiation from the same sample. This presents problems in that if the detector is exposed to radiation for a sufficient time to generate accurately measurable charge from extremely feeble radiation, those parts of the detector that are exposed to extremely intense radiation will have accumulated excessive charge. Charge accumulation is excessive when it exceeds the capacity of the device to store it. Not only is such excessive charge useless for measurement of the intensity of the radiation that generated it, but it can also spill over into adjacent regions of the detector and impede or prevent the correct functioning of those regions. Such a process is known as xe2x80x98bloomingxe2x80x99. Conversely, if the detector is exposed for the short period of time appropriate for the measurement of extremely intense radiation, those regions of the detector that are exposed only to feeble radiation will not accumulate sufficient charge to allow accurate measurement.
According to a first aspect of the invention, there is provided a method for spectrochemical analysis of a sample comprising
(i) interacting a representative portion of the sample and a spectroscopic radiation source to produce spectral radiation of the sample,
(ii) measuring the intensity of the spectral radiation as a function of wavelength by passing the spectral radiation through a polychromator onto a multielement solid state detector and reading a plurality of elements of the detector, including
(iii) exposing the detector to the spectral radiation for a plurality of exposure times of varying duration whereby for short exposure times charge accumulation in elements of the detector due to high intensity components of the spectral radiation is limited and for longer exposure times charge accumulation in elements of the detector due to feeble intensity components of the spectral radiation is increased,
(iv) obtaining a separate data set for each exposure time such that measurement data at a wavelength at which the intensity is high is obtainable from a data set collected for a short duration exposure time and measurement data at a wavelength at which the intensity is low is obtainable from a data set collected for a relatively long duration exposure time, and
(v) extracting spectrochemical information about the sample from measurement data selected from different ones of the separate data sets.
The invention also provides, in a second aspect, spectroscopy apparatus for spectrochemical analysis of a sample comprising
a spectroscopic radiation source and a system for interacting the radiation source and a sample for providing spectral radiation of the sample,
an optical spectrometric system including a polychromator and a multi-element solid state detector for providing intensity measurements of the spectral radiation as a function of wavelength and means for reading a plurality of elements of the detector to provide said intensity measurements,
and a device that is operable on application of an electrical signal thereto to prevent or to allow transmission of the spectral radiation to the detector, wherein the device is selectively operable for the detector to be exposed to the spectral radiation for a plurality of exposure times of varying duration.
The radiation source may be adapted to receive a representative portion of an analytical sample and to heat this portion to a temperature sufficient to decompose it and to excite spectrochemical emission of light from molecules, atoms or ions resulting from the decomposed portion, for example as in atomic emission spectrometry. Alternatively light from the radiation source may be passed through a decomposed sample portion and its absorption at particular wavelengths measured, for example as in atomic absorption spectrophotometry. Other techniques encompassed by the invention include passage of radiation through a suitably presented sample and measurement of its absorption at particular wavelengths (for example as in ultra-violet-visible absorption spectrometry, or infrared absorption spectrophotometry), or measurement of emitted light at particular wavelengths (for example as in fluorescence spectrophotometry). The wording herein of interacting a radiation (or light) source and a sample for providing xe2x80x9cspectral radiation (or light) of the samplexe2x80x9d is intended to encompass all of these and other similar spectrochemical analysis techniques involving the measurement of the intensity of resultant radiation as a function of wavelength.
The detector of the second aspect of the invention consists of an array of detection elements that can be read by passing charge from one optically sensitive detection element (or pixel) to the immediately adjacent optically sensitive detection element (or pixel) in the same column while the detector is not exposed to light. Means are provided for minimising the spread of charge from any given pixel to all adjacent pixels. This reduces the detrimental effects of any spread of charge (xe2x80x98bloomingxe2x80x99) that result from the exposure of a pixel to excessively intense radiation. Preferably the detector is oriented in such a way that the effect of blooming is restricted to pixels that would be exposed to radiation in the same spectral order as the radiation that caused the blooming. Any blooming that may occur is thus manifested as a spreading of an intense spectral line rather than as an unexpected increase in intensity at a remote wavelength. Preferably the detector is of the type known as a xe2x80x98megapixel arrayxe2x80x99, having a large number of pixels (nominally 1 million) and preferably the optical system is so configured that when an optical spectrum is projected onto the detector by said optical system there is a plurality of pixels in the area of the detector that receives each spectral feature of interest (for example each atomic or ionic emission line in an emission spectrometric measurement). In the manufacturing process of array detectors, it is to be expected that a variable number of pixels in each detector will be defective. If there be a plurality of pixels in the area upon which any specific emission line is imaged, a detector will be functional even if one of said plurality of pixels at said position is defective. This means that the yield of useful detectors from the detector manufacturing process is greater than it would be if, for example, only one pixel were provided at each position at which a spectral line were imaged. This greater yield of useful detectors has the effect of reducing the unit cost of the detectors. Preferably too the detector is back-thinned and back illuminated, as described for example in xe2x80x98Charge Transfer Devices in Spectroscopyxe2x80x99, J. V. Sweedler, K. L. Ratslaff and M. B. Denton, eds., VCH Publishers, Inc., New York, 1994, to provide efficient detection of ultraviolet light. Preferably the detector is responsive to light having wavelengths ranging from the far ultraviolet (for example 175 nm) to the far red end of the optical spectrum (for example 785 nm).
To allow the accurate measurement of the wide range of intensities of radiation that are required for the spectrochemical analysis of analytical samples, each reading of the intensity of light at the wavelengths required for a spectrochemical measurement consists of a sequence of exposures of varying duration. This is to ensure that each reading includes at least one exposure in which the amount of charge accumulated at each wavelength of interest is neither too little nor too great. The duration of each exposure of the detector is controlled by the period of time for which an optical shutter or other equivalent device is maintained in a state that allows radiation from the source to fall on the detector.
Preferably, to increase further the range of intensities of radiation that can be measured, at least one exposure is made with an optical attenuator placed between the source and the detector. The use of an attenuator is desirable in that it permits longer exposure times to be used for the measurement of intense radiation. Long exposure times are advantageous in that they result in the averaging out of any periodic variations in the intensity of incident radiation that may be introduced, for example, by the action of a pump used to transport a liquid sample into a radiation (i.e. an excitation) source such as an electrical plasma.
The set of data from each exposure is stored separately. Measurement at a wavelength at which the intensity of radiation is high is obtained from a data set collected with a relatively short measurement time and with the optical attenuator placed between the source and the detector. Measurement at a wavelength at which there is only a low intensity of radiation is obtained from a data set collected with a relatively long exposure time and without the optical attenuator between the source and the detector. Measurements at wavelengths at which there is an intermediate intensity of radiation is obtained from data sets having intermediate exposure times. The selection of appropriate data is done automatically, using pre-set criteria to choose data corresponding to an appropriate amount of charge accumulation.
Thus the method of the invention includes exposing the detector to radiation from the light (radiation) source for a plurality of exposure times of varying duration, said duration being established by the period of time for which the optical shutter or other equivalent selectable light-excluding means allows light from the source to fall on the detector. Preferably at least one such exposure is made with an optical attenuator, which preferably reduces the transmission of light by factors of 10 to 100, placed between the light source and the detector. Data from each exposure is stored and analysed and only data corresponding to an appropriate level of charge accumulation is taken for further processing to extract spectrochemical information about the sample.
For a better understanding of the invention and to show how it may be carried into effect, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.